How many miles is on a car if it says it has 75k on it?

75 000, probably, though it might mean 75 000 km, which is 46602.8394 miles (according to Google's converter program).

"k" stands for "kilo" - a prefix used in metric measurements to refer to 1000. Hence 1 kilometre is 1000 metres, etc. Used without a unit, as you have provided, it simply abbreviates the "000" at the end of the number. However, since it is a metric prefix, the vehicle may record milage in kilometres rather than miles

The easiest way to find out is to use the car's distance measuring system (resettable milage indicator for company trips, etc - most cars have them) and drive a known distance (say, if you know the distance from home to work is 10 miles. Measure that distance, and if you get "10" recorded, it's in miles. If you get a larger number (16, probably) it's in kilometres.

Or you could track down where the car was made - if its target market is not the US, it's probably measuring kilometres.